A Josephson tunnel junction usually comprises a thin tunnel barrier layer sandwiched between two thin superconducting metallic films. The voltage across a Josephson junction is capable of rapidly switching from zero volts to a finite voltage when a critical current passes through the junction. As a result, Josephson devices are currently under consideration for use in logic and memory circuits.
Typically, a conventional Josephson junction may be formed by thermally evaporating a lead or other superconducting film onto a substrate in an evacuable chamber. The tunnel barrier may be formed by oxidizing the exposed surface of the lead film. A second lead film is then thermally evaporated onto the oxidized surface. The superconducting film contiguous with the substrate is known as the base electrode and the other superconducting film is known as the counter electrode.
It is known from the prior art that hillock formation on the electrodes is deleterious to the operation of conventional Josephson tunnel junctions of the type previously described. Hillocks are protrusions which develop on the surface of metallic thin films that are subjected to thermal cycling between temperature extremes (see Chaudari et al., U.S. Pat. No. 4,012,756, issued on Mar. 15, 1977). The superconducting films forming the base and counter electrodes in conventional Josephson tunnel junctions are susceptible to the formation of hillocks when the Josephson junctions are cycled between superconducting temperatures, on the order of 4 degrees Kelvin, and room temperatures, on the order of 300 degrees Kelvin, or photolithographic processing temperatures, on the order of 360 degrees Kelvin. Hillocks which form on the base electrode of a Josephson junction may protrude through the tunnel barrier and short circuit the junction. Accordingly, efforts have been directed to trying to find hillock-resistant superconducting films suitable for use in sueprconducting devices such as Josephson tunnel junctions.
The voltage which develops across a Josephson junction device is characteristic of the materials comprising the base and counter electrodes. In the case of the previously mentioned lead-lead oxide-lead Josephson tunnel junction, the voltage is approximately equal to twice the energy gap of lead or about 2.5 millivolts. Thus, in order to utilize conventional Josephson junction technology for the formation of high voltage superconducting switches, it is necessary to connect a plurality of Josephson junctions in series. This usually involves numerous deposition steps or a complicated masking procedure (see Hebard, A. F. et al., "Diagnostics With Series Connected Josephson Tunnel-Junctions", Journal of Applied Physics, Vol. 49, No. 1, January 1978, p. 338). It would clearly be advantageous to produce series connected Josephson tunnel junctions in a single deposition step.
In addition to the previously mentioned thermal evaporation technique for forming lead films, other techniques for the formation of lead or other superconducting metal films are taught in the prior art. One such technique involves the reactive ion beam-sputter deposition of a metal in the presence of oxygen (see Castellano, R. N., "Reactive Ion Beam Sputtering of Thin Films of Lead, Zirconium, and Titanium", Thin Solid Films, Vol. 46, 1977, pp. 213-221).
Castellano directed a beam of energetic ions at a metal target in an evacuable chamber having an atmosphere of oxygen. By a momentum transfer mechanism atoms on or near the surface of the target were ejected and deposited on a suitable substrate as a thin film comprising metal and metal oxide. Castellano studied the deposition rate, resistivity, and internal stress of the films as a function of the partial pressure of oxygen which was varied from approximately 1.times.10.sup.-6 Torr to approximately 2.times.10.sup.-4 Torr, but did not propose fabrication of Josephson junctions or other superconducting devices from the films.